The invention is in the field of photoflash lamps of the miniature type in which a pair of lead-in wires carry a filament or other electrical ignition means inside a bulb having a volume less than two cubic centimeters and containing combustion material such as shredded metal foil and a combustion-supporting gas such as oxygen. The lead-in wires extend into the bulb through a seal at the base of the bulb, and the lamps are conventionally positioned base-down when flashed in flashcubes and multiple flash arrays.
When such a photoflash lamp is flashed, the shredded metal foil burns in an oxygen atmosphere and produces an intense flash of light, accompanied by intense heat which melts back the lead-in wires from the ignition means. Occasionally, some remnants of metal will remain in a flashed lamp and tend to bridge across the melted-back wires, causing an electrically shorted flashed lamp. Also, the melted-back lead-in wires sometimes become distorted and touch one another, and/or melt and fuse together, causing a directly contacting short with one another. This shorting is undesirable in certain applications, such as in flash arrays like the FlashBar type of array in which flashed lamps must be non-shorted in order that the flash sequencing can function properly to cause flashing of subsequent lamps in the array.
Due to the small size of miniature flash lamps, it is not feasible to provide an internal glass stem to seal and support the lead-in wires within the bulb as had been the case with previous large-bulb flash lamps, because such an internal stem would be difficult to construct in such a small bulb and would considerably reduce the amount of space available for the oxygen. Therefore, miniature types of electrically fired flash lamps are conventionally constructed with a pair of lead-in wires sealed through a press-seal at the base of the bulb, and in some types, a small glass bead is sealed to the lead-in wires within the bulb to provide mechanical support and to facilitate the manufacturing process.
Various ways have been devised for making miniature electrically fired flash lamps which will be open-circuited, or at least not short-circuited, between the lead-in wires after having been flashed. U.S. Pat. No. 3,816,054 to Baldrige and Sobieski discloses a flash lamp having electrical insulation means, such as a glass sleeve around one of the lead-in wires, to provide an increased electrical path length between the melted-back lead-in wires after the lamp is flashed. U.S. Pat. Nos. 3,897,196 and 3,919,750 to Saunders and Leach disclose a flash lamp having a glass bead fused around the lead-in wires, the bead being pushed up at its bottom to provide a raised barrier at its top between the lead-in wires. U.S. Pat. No. 3,941,555 to Anderson and Sobieski discloses a flash lamp having a bead sealed to the lead-in wires and shaped to have sloping sides and a narrow ridge at the top extending between the lead-in wires, to prevent after-flash shorting by unburned metal strands deposited on the bead.